By now, BART’s Millbrae Station — with its connections to Caltrain, buses and shuttles — was supposed to see 16,500 passengers pass through its fare gates every weekday, making it the fifth-busiest station in the system.

But 13 years after it opened, it’s pretty much just another BART station, nowhere near the bustling Peninsula transit hub that planners envisioned. With just under 7,000 riders entering and exiting daily, the station ranks 27th in activity among the transit system’s 46 stations.

BART and Caltrain officials acknowledge Millbrae Station isn’t what planners had in mind when they opened it in 2003 as part of a five-stop extension that would take the rail system to San Francisco International Airport. But, even so, they say it’s still a vital link between two popular pieces of the Bay Area transit network.

“It works — for the people who use it,” said Joel Keller, a veteran BART director who was on the board as the station was being planned, debated and built.

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But there’s no question that the ridership projections — and hopes — for the station were overly optimistic.

“The models we have are pretty good,” Keller said. “But I always look at the models with some skepticism because people will make decisions based on what’s best for them.”

And that’s what happened. When ridership forecasts were made in 1995, the dot-com boom had just begun and the economy was surging. A growing economy meant more jobs and employees who needed a way to get to them. By the time the station opened, the boom had gone bust. And by the time the economy recovered, where people worked had changed.

Forecasters made other assumptions that never came true. They thought Caltrain fares would be raised, giving riders an economic incentive to transfer to BART, that BART trips between Millbrae and San Francisco would be faster than they are and that Caltrain would end most of its northbound trains at Millbrae, forcing passengers to switch to BART to get downtown.

It hasn’t helped that transfers between BART and Caltrain turned out to be awkward and unreliable and that it’s not easy for riders who want to get to the airport on BART from Millbrae. While passengers from the north can take BART directly to the airport, passengers from the south usually have to hop on BART at Millbrae, get off at San Bruno Station and transfer to an airport train.

Expectations for the new station were high.

“It’s the big one,” Molly McArthur, a BART spokeswoman, now retired, said at the time. “It’s going to be a hive of activity. It’s really more than a BART station, it’s a transit hub. And that’s really a new thing for us at BART.”

As the largest transit station in the Western U.S., Millbrae is indeed big. It’s surrounded by parking lots and a five-story parking structure — nearly 3,000 spaces in all — and has five tracks: three for BART and two for Caltrain. The tracks are all at ground level, but the station concourse with ticket machines and restrooms are above ground.

BART riders coming from the north and bound for Caltrain arrive on the east side of the station. To catch Caltrain, they have to ascend escalators or stairs, exit through the BART fare gates and walk a few yards across the concourse before descending to the Caltrain station’s west side.

Northbound Caltrain riders planning to take BART into San Francisco have an easier time of it. They can avoid the stairs and escalators by passing through a set of BART fare gates for a quicker transfer to BART.

Riders also can be foiled by a lack of coordination between the two rail systems. Trains often run late, and sometimes BART arrives just minutes before Caltrain is scheduled to depart.

BART and Caltrain officials say they talk to each other occasionally about coordinating schedules but have developed no official policies to ensure a more consistent and seamless transfer.

“We do try to coordinate arrivals so as not to leave people stranded at the station, looking at the red lights of a departing train,” Keller said. “But it doesn’t always work.”

Photo: Leah Millis, The Chronicle

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A BART train pulls away from Millbrae Station. Thirteen years after the station opened, ridership at the hub that connects BART and Caltrain hasn’t been what planners envisioned.

A BART train pulls away from Millbrae Station. Thirteen years after the station opened, ridership at the hub that connects BART and Caltrain hasn’t been what planners envisioned.

Photo: Leah Millis, The Chronicle

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Riders sleep on a San Francisco-bound train at the Millbrae BART Station.

Riders sleep on a San Francisco-bound train at the Millbrae BART Station.

Photo: Leah Millis, The Chronicle

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Passengers walk to the entrance of a San Jose-bound train at the Millbrae BART Station.

Passengers walk to the entrance of a San Jose-bound train at the Millbrae BART Station.

Photo: Leah Millis, The Chronicle

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A pedestrian exits the Millbrae BART Station while passengers board a San Jose-bound train.

A pedestrian exits the Millbrae BART Station while passengers board a San Jose-bound train.

Photo: Leah Millis, The Chronicle

Millbrae BART Station ridership isn’t near original expectations

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Tasha Bartholomew, a Caltrain spokeswoman, said the agency shares its draft schedules with BART and asks for suggestions. When things don’t go as planned, engineers will hold a train for scurrying passengers while trying to stay on schedule.

“While we try to accommodate courtesy connections at Millbrae, it is not always possible,” she said. “We are two separate operations with different signal systems and infrastructure.”

Nina Leviten, 18, who lives in Palo Alto and attends classes at UC Berkeley, said she takes Caltrain to Millbrae, transfers to BART and rides to downtown Berkeley. On the way home, she reverses the trip.

The joint station at Millbrae makes it possible for her to attend Cal, she said, but sometimes the connections between trains are tough, especially on her way home when she has to rush up and down stairs.

“It’s a little difficult because (Caltrain) leaves just three minutes after BART arrives,” she said. “Last week I missed it two times and had to wait 20 minutes.”

While the projected ridership still hasn’t materialized for Millbrae Station, BART and Caltrain officials remain optimistic that it will grow.

A major transit-oriented development with 376 units of housing, a 150-room hotel, and office and retail space is planned on land next to the station. Meanwhile, Caltrain is proceeding with plans to electrify its system, increasing the number of trains it can run. And, someday, the state’s high-speed rail line is expected to share the Caltrain tracks, with stops at Millbrae.

“It’s always been an aspirational station where we look for increased ridership,” Keller said. “Eventually, it has the potential to be a much busier station.”